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Low-Cost Spectrometer Incorporates Spherical Beam Volume Hologram

Photonics Spectra
Apr 2006
Replacing grating offers economic benefits without sacrificing resolution.

Hank Hogan

By exploiting a spherical beam volume hologram, researchers at Georgia Institute of Technology in Atlanta have devised an inexpensive spectrometer without sacrificing resolution. Ali Adibi, an associate professor in the School of Electrical and Computer Engineering and the team leader, expects the development to reduce the cost of such devices by at least a factor or four or five.

The key to the cost reduction, which also would be accompanied by a decrease in size and an increase in robustness, is the replacement of the thin grating in such devices with the volume hologram. The grating often is the most expensive component of such an instrument.

In the Georgia Tech approach, a spherical beam volume hologram handles the functions of the slit, first or collimating lens, grating and second lens in a conventional spectrometer. The hologram is built on a narrow sheet of photopolymer through the interference of a plane wave that arrives at an angle and a spherical beam that arises from a point source. When incoming light falls on the hologram from the same direction as the spherical beam did, the result is a diffracted beam that can be read by a CCD.

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The lensless spectrometer was constructed using a spherical beam volume hologram and a low-cost Web camera. The prototype measures 6 × 2 × 2 cm and can be made smaller. Production devices will be black to avoid scattering light.

Although the technique does not offer any advantages in efficiency over that of the conventional slit approach, it is more forgiving. A misalignment between the components of a traditional spectrometer leads to a loss of light and a diminished signal. The hologram method requires no special alignment.

Using a spherical beam volume hologram also makes it possible to implement multimode multiplex spectroscopy, which gets around the trade-off between resolution and throughput. In a conventional spectrometer, this can be done with multiple slits: The use of narrow slits improves the resolution, and having many of them boosts throughput. The multiple-slit arrangement can be complicated to build, however. In the new approach, only the multiplexing of several holograms in one piece of plastic would be required.

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The rainbow crescent shows the output of a beam transmitted through the spherical beam volume hologram and read by white light at the back of the hologram. The color of the crescent in the transmitted beam is the complement of the color in the diffracted crescent.

There are limitations to using a spherical beam volume hologram, primarily because the thickness of the photopolymer determines the resolution. Adibi said that the resolution currently is approximately 2 to 4 nm because the thickest commercially available polymer with the required optical quality has a thickness of approximately 400 µm. However, he pointed out that progress is being made in this area by Aprilis Inc. of Maynard, Mass.

Adibi said that one application of the spectrometers is in a system for blood alcohol characterization. His group developed the spectrometer for such a system as part of a National Institute on Alcohol Abuse and Alcoholism project led by David Brady of Duke University in Durham, N.C.


GLOSSARY
grating
A framework or latticework having an even arrangement of rods, or any other long narrow objects with interstices between them, used to disperse light or other radiation by interference between wave trains from the interstices. The ability of a grating to separate wavelengths (chromatic resolving power) is expressed as being equal to the number of lines in the grating.
spectrometer
A kind of spectrograph in which some form of detector, other than a photographic film, is used to measure the distribution of radiation in a particular wavelength region.  
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